In recent years, there has been a rapid progress in the development of small-format sensors harnessing the MEMS (micro-electromechanical systems) technology, and sensors adapted to sense physical quantities such as acceleration have been used in various applications inclusive of cellular phones and game machines, with their applications now under study. Such sensors are fabricated using an SOI wafer having a triple-layer structure of, for instance, a silicon layer/silicon oxide layer/silicon layer. A typical sensor comprises a frame having an opening formed in such a way as to hollow out an SOI wafer, a displaceable weight supported on this frame via a plurality of beams and a piezoelectric-resistor located at each beam. And as external force is applied to the weight to trigger off displacement, it causes the beams to bend in association with that displacement, and the resistance of the piezoelectric-resistor located at the beam to change incidental to the amount of bending of the beam. Then, that resistance change is sensed to sense physical quantities such as acceleration.
To make the sensitivity of such sensors high, it is preferable that a short piezoelectric-resistor is located at a stress concentration site. However, a problem with this is that only with the use of the short piezoelectric-resistor, there are increasing power consumptions, because voltage is applied to the piezoelectric-resistor during driving. To solve that problem, there is now a sensor developed wherein a plurality of short piezoelectric-resistors are connected in series by a high-concentration diffusion layer (JP(A) 2006-98321).
In the sensor set forth in JP(A) 2006-98321, however, each piezoelectric-resistor is formed by diffusing a low concentration of impurities in a silicon substrate, and the high concentration diffusion layer adapted to connect a plurality of piezoelectric-resistors in series, too, is a silicon substrate with a high concentration of impurities diffused in it; so there is Joule heat generated from the piezoelectric-resistors and high concentration diffusion layer by the voltage applied to the piezoelectric-resistor during driving. Further in the sensor disclosed in JP(A) 2006-98321, each piezoelectric-resistor and the high concentration diffusion layer are covered with an insulating layer; in other words, that sensor has a structure less likely to release off heat. For this reason, the generated Joule heat gives rise to thermal expansion of the beams, wirings located on the beams, the silicon oxide layer, and the silicon layer. This in turn triggers off a temporal change in the output value of the sensor, offering a problem that the sensor's reliability goes worse.
The sensor disclosed in JP(A) 2006-98321 is fabricated by a process comprising (1) the step of diffusing a low concentration of impurities in a silicon substrate to form a piezoelectric-resistor, (2) the step of diffusing a high concentration of impurities in the silicon substrate to form a high-concentration diffusion layer, (3) the step of providing an insulating layer over the piezoelectric-resistor and high-concentration diffusion layer and providing a contact hole in it, and (4) the step of forming wirings: the need for four cycles of photolithography results in low productivity. Another problem is that wirings for electrically connecting multiple piezoelectric-resistors in series lie on the high-concentration diffusion layer alone, and as defects occur at this side, it directly means malfunction of the sensor itself.